Application of anti-Sclerostin therapy in non-osteoporosis disease models

SOST gene suppression stimulates osteocyte Wnt/β-catenin signaling to prevent bone resorption and attenuates particle-induced osteolysis

The most common cause for prosthetic revision surgery is wear particle-induced periprosthetic osteolysis, which leads to aseptic loosening of the prosthesis. Both SOST gene and its synthetic protein, sclerostin, are hallmarks of osteocytes. According to our previous findings, blocking SOST induces bone formation and protects against bone loss and deformation caused by titanium (Ti) particles by activating the Wnt/β-catenin cascade. Although SOST has been shown to influence osteoblasts, its ability to control wear-particle-induced osteolysis via targeting osteoclasts remains unclear. Mice were subjected to development of a cranial osteolysis model. Micro CT, HE staining, and TRAP staining were performed to evaluate bone loss in the mouse model. Bone marrow-derived monocyte-macrophages (BMMs) made from the C57BL/6 mice were exposed to the medium of MLO-Y4 (co-cultured with Ti particles) to transform them into osteoclasts. Bioinformatics methods were used to predict and validate the interaction among SOST, Wnt/β-catenin, RANKL/OPG, TNF-α, and IL-6. Local bone density and bone volume improved after SOST inhibition, both the number of lysis pores and the rate of skull erosion decreased. Histological research showed that β-catenin and OPG expression were markedly increased after SOST inhibition, whereas TRAP and RANKL levels were markedly decreased. In-vitro, Ti particle treatment elevated the expression of sclerostin, suppressed the expression of β-catenin, and increased the RANKL/OPG ratio in the MLO-Y4 cell line. TNF-α and IL-6 also elevated after treatment with Ti particles. The expression levels of NFATc1, CTSK, and TRAP in osteoclasts were significantly increased, and the number of positive cells for TRAP staining was increased. Additionally, the volume of bone resorption increased at the same time. In contrast, when SOST expression was inhibited in the MLO-Y4 cell line, these effects produced by Ti particles were reversed. All the results strongly show that SOST inhibition triggered the osteocyte Wnt/β-catenin signaling cascade and prevented wear particle-induced osteoclastogenesis, which might reduce periprosthetic osteolysis. KEY MESSAGES: SOST is a molecular regulator in maintaining bone homeostasis. SOST plays in regulating bone homeostasis through the Wnt/β-catenin signaling pathway. SOST gene suppression stimulates osteocyte Wnt/β-catenin signaling to prevent bone resorption and attenuates particle-induced osteolysis.

Reduction of SOST gene promotes bone formation through the Wnt/β-catenin signalling pathway and compensates particle-induced osteolysis

The increase in bone resorption and/or the inhibition of bone regeneration caused by wear particles are the main causes of periprosthetic osteolysis. The SOST gene and Sclerostin, a protein synthesized by the SOST gene, are the characteristic marker of osteocytes and regulate bone formation and resorption. We aimed to verify whether the SOST gene was involved in osteolysis induced by titanium (Ti) particles and to investigate the effects of SOST reduction on osteolysis. The results showed osteolysis on the skull surface with an increase of sclerostin levels after treated with Ti particles. Similarly, sclerostin expression in MLO-Y4 osteocytes increased when treated with Ti particles in vitro. After reduction of SOST, local bone mineral density and bone volume increased, while number of lytic pores on the skull surface decreased and the erodibility of the skull surface was compensated. Histological analyses revealed that SOST reduction increased significantly alkaline phosphatase- (ALP) and osterix-positive expression on the skull surface which promoted bone formation. ALP activity and mineralization of MC3T3-E1 cells also increased in vitro when SOST was silenced, even if treated with Ti particles. In addition, Ti particles decreased β-catenin expression with an increase in sclerostin levels, in vivo and in vitro. Inversely, reduction of SOST expression increased β-catenin expression. In summary, our results suggested that reduction of SOST gene can activate the Wnt/β-catenin signalling pathway, promoting bone formation and compensated for bone loss induced by Ti particles. Thus, this study provided new perspectives in understanding the mechanisms of periprosthetic osteolysis.

The role of osteoblasts in energy homeostasis

Osteoblasts are specialized mesenchymal cells that synthesize bone matrix and coordinate the mineralization of the skeleton. These cells work in harmony with osteoclasts, which resorb bone, in a continuous cycle that occurs throughout life. The unique function of osteoblasts requires substantial amounts of energy production, particularly during states of new bone formation and remodelling. Over the last 15 years, studies have shown that osteoblasts secrete endocrine factors that integrate the metabolic requirements of bone formation with global energy balance through the regulation of insulin production, feeding behaviour and adipose tissue metabolism. In this article, we summarize the current understanding of three osteoblast-derived metabolic hormones (osteocalcin, lipocalin and sclerostin) and the clinical evidence that suggests the relevance of these pathways in humans, while also discussing the necessity of specific energy substrates (glucose, fatty acids and amino acids) to fuel bone formation and promote osteoblast differentiation.

Osteoporosis-pseudoglioma syndrome: clinical, genetic, and treatment-response study of 10 new cases in Greece

Osteoporosis-pseudoglioma syndrome (OPPG) is a rare autosomal-recessive disorder, characterized by severe osteoporosis and early-onset blindness. Loss of function mutations in the gene encoding low-density lipoprotein receptor-related protein 5 (LRP5) have been established as the genetic defect of the disease. We report the clinical and genetic evaluation of ten OPPG cases in eight related nuclear families and their close relatives. Bone mineral density (BMD) in OPPG patients was assessed by dual-energy X-ray absorptiometry (DXA). Genotyping of LRP5 gene and targeted detection of index mutation were performed by DNA direct sequencing. Four patients were introduced to bisphosphonates. Mutational screening of LRP5 gene revealed the c.2409_2503+79del deletion in homozygous state, expected to result in a truncated protein. Among 44 members of the pedigree, 10 (22%) were identified homozygous and 34 (59%) heterozygous for this mutation. All patients had congenital blindness and 7 of them had also impaired bone mineral density. Four of them received bisphosphonates and responded with decreased bone pain and improvement in BMD; however, 3 patients presented with one fracture during treatment.Conclusion: The current study presents the molecular and clinical profiles of 10 new OPPG cases, being part of an extended pedigree. Patients who received bisphosphonate treatment responded well with increase in their BMD, though fractures occurred during therapy. What is known: • OPPG syndrome is a rare genetic disorder characterized by congenital blindness and juvenile osteoporosis. • Loss of function mutations in the gene encoding low-density lipoprotein receptor-related protein 5 (LRP5) is the genetic defect of the disease. What is new: • Genetic and clinical phenotype of 10 new OPPG patients. • The ten new OPPG patients presented with phenotypical variability in osseous manifestations.

A novel nonosteocytic regulatory mechanism of bone modeling

Osteocytes, cells forming an elaborate network within the bones of most vertebrate taxa, are thought to be the master regulators of bone modeling, a process of coordinated, local bone-tissue deposition and removal that keeps bone strains at safe levels throughout life. Neoteleost fish, however, lack osteocytes and yet are known to be capable of bone modeling, although no osteocyte-independent modeling regulatory mechanism has so far been described. Here, we characterize a novel, to our knowledge, bone-modeling regulatory mechanism in a fish species (medaka), showing that although lacking osteocytes (i.e., internal mechanosensors), when loaded, medaka bones model in mechanically directed ways, successfully reducing high tissue strains. We establish that as in mammals, modeling in medaka is regulated by the SOST gene, demonstrating a mechanistic link between skeletal loading, SOST down-regulation, and intense bone deposition. However, whereas mammalian SOST is expressed almost exclusively by osteocytes, in both medaka and zebrafish (a species with osteocytic bones), SOST is expressed by a variety of nonosteocytic cells, none of which reside within the bone bulk. These findings argue that in fishes (and perhaps other vertebrates), nonosteocytic skeletal cells are both sensors and responders, shouldering duties believed exclusive to osteocytes. This previously unrecognized, SOST-dependent, osteocyte-independent mechanism challenges current paradigms of osteocyte exclusivity in bone-modeling regulation, suggesting the existence of multivariate feedback networks in bone modeling-perhaps also in mammalian bones-and thus arguing for the possibility of untapped potential for cell targets in bone therapeutics.

High Fidelity of Mouse Models Mimicking Human Genetic Skeletal Disorders

The 2019 International Skeletal Dysplasia Society nosology update lists 441 genes for which mutations result in rare human skeletal disorders. These genes code for enzymes (33%), scaffolding proteins (18%), signal transduction proteins (16%), transcription factors (14%), cilia proteins (8%), extracellular matrix proteins (5%), and membrane transporters (4%). Skeletal disorders include aggrecanopathies, channelopathies, ciliopathies, cohesinopathies, laminopathies, linkeropathies, lysosomal storage diseases, protein-folding and RNA splicing defects, and ribosomopathies. With the goal of evaluating the ability of mouse models to mimic these human genetic skeletal disorders, a PubMed literature search identified 260 genes for which mutant mice were examined for skeletal phenotypes. These mouse models included spontaneous and ENU-induced mutants, global and conditional gene knockouts, and transgenic mice with gene over-expression or specific base-pair substitutions. The human X-linked gene ARSE and small nuclear RNA U4ATAC, a component of the minor spliceosome, do not have mouse homologs. Mouse skeletal phenotypes mimicking human skeletal disorders were observed in 249 of the 260 genes (96%) for which comparisons are possible. A supplemental table in spreadsheet format provides PubMed weblinks to representative publications of mutant mouse skeletal phenotypes. Mutations in 11 mouse genes (Ccn6, Cyp2r1, Flna, Galns, Gna13, Lemd3, Manba, Mnx1, Nsd1, Plod1, Smarcal1) do not result in similar skeletal phenotypes observed with mutations of the homologous human genes. These discrepancies can result from failure of mouse models to mimic the exact human gene mutations. There are no obvious commonalities among these 11 genes. Body BMD and/or radiologic dysmorphology phenotypes were successfully identified for 28 genes by the International Mouse Phenotyping Consortium (IMPC). Forward genetics using ENU mouse mutagenesis successfully identified 37 nosology gene phenotypes. Since many human genetic disorders involve hypomorphic, gain-of-function, dominant-negative and intronic mutations, future studies will undoubtedly utilize CRISPR/Cas9 technology to examine transgenic mice having genes modified to exactly mimic variant human sequences. Mutant mice will increasingly be employed for drug development studies designed to treat human genetic skeletal disorders.

SIGNIFICANCE

Great progress is being made identifying mutant genes responsible for human rare genetic skeletal disorders and mouse models for genes affecting bone mass, architecture, mineralization and strength. This review organizes data for 441 human genetic bone disorders with regard to heredity, gene function, molecular pathways, and fidelity of relevant mouse models to mimic the human skeletal disorders. PubMed weblinks to citations of 249 successful mouse models are provided.

Targeting WNT signaling in the treatment of osteoporosis

Osteoporosis is a widespread chronic disease characterized by low bone density, altered microstructure and bone fragility, leading to low impact fractures in affected individuals. The discovery of a few mutations that cause extremely rare human diseases has identified the WNT signaling pathway as a candidate for therapeutic intervention aimed at increasing bone mass and strength. In particular, inhibition of sclerostin, a WNT antagonist secreted by osteocytes, has proven in clinical trials to be a very efficient osteo-anabolic approach. One year of monthly administration of antibodies to sclerostin rapidly decreases bone resorption and increases bone formation and bone density at all sites, decreasing markedly fracture risk in treated patients. Their effect is however limited in time and cardiovascular adverse events have been reported in one clinical trial.

Computational and functional characterization of four SNPs in the SOST locus associated with osteoporosis

The SOST gene encodes sclerostin, a C-terminal cysteine knot-like domain containing key negative regulator of osteoblastic bone formation that inhibits LRP5/6-mediated canonical Wnt signaling. Numerous single nucleotide polymorphisms (SNPs) in the SOST locus are firmly associated with bone mineral density (BMD) and fracture in genome-wide association studies (GWAS) and candidate gene association studies. However, the validation and mechanistic elucidation of causal genetic variants, especially for SNPs located beyond the promoter-proximal region, remain largely unresolved. By employing computational and experimental approaches, here we identify four SNPs rs1230399, rs7220711, rs1107748 and rs75901553 as functional variants which display allelic variation in SOST gene expression. The osteoporosis associated SNP rs1230399 in the SOST distal upstream regulatory region shows FOXA1 binding activity with subsequent transinactivation in a T allele-specific manner. The BMD GWAS lead SNPs rs7220711 and rs1107748 both reside in the 52-kb regulatory element deletion 35-kb downstream of the SOST gene which leads to Van Buchem disease. The rs7220711-A has a higher affinity for the transcriptional repressors MAFF or MAFK homodimers than rs7220711-G, while rs1107748 confers C allele specific transcriptional enhancer activity via a CTCF binding element. The variant rs75901553 C>T located in a conserved site of the SOST 3' UTR abolishes a target binding site for miR-98-5p which is negatively responsive to parathyroid hormone or 17β-estradiol in osteoblastic cell lines. Our findings uncover the biological consequences of four independent genetic variants in the SOST region and their important roles in SOST expression via diverse mechanisms, providing new insights into the genetics and molecular pathogenesis of osteoporosis.

Targeted inhibition of sclerostin for post-menopausal osteoporosis therapy: A critical assessment of the mechanism of action

Promising news in the treatment of osteoporosis is that sequestering sclerostin from circulation with antibodies stimulates robust bone formation. Pre-clinical studies on rodents and monkeys have confirmed that treatment with anti-sclerostin monoclonal antibody (Scl-Ab) increases bone mass, improves bone strength and enhances fracture repair. Clinical trials show that bone gain (anabolic effect) is transient and are primarily at central (spine and hips) than peripheral (wrist) sites. Interestingly Scl-Ab also inhibited bone resorption. Thus Scl-Ab is being regarded as the pharmacologic agent with dual properties - stimulating bone formation and decreasing bone resorption. Sclerostin neutralization transiently increases bone formation markers in post-menopausal women and like parathyroid hormone (PTH) activates osteoblasts and lining cells resulting in bone anabolic effect. However, unlike PTH, sclerostin antibody also decreases bone resorption (anti-catabolic). Although, the U.S. Food and Drug Administration have accepted the Biologics License Application for one of the monoclonal antibodies against sclerostin (romosozumab) for review, many questions remain before romosozumab can be introduced as a skeletal anabolic agent to clinical practice. For example, neutralizing sclerostin alters calcium homeostasis and increases PTH. In addition, sclerostin depletion in preclinical studies has been reported to severely compromises B cell depletion in bone marrow. We have reviewed the currently available evidences that support the use of sclerostin antibody in treating osteoporosis and compare its efficacy and mechanism of action with the currently available anabolic drug, human PTH.